1. Field of the Invention
The present invention relates to toric contact lenses, and more particularly to toric contact lenses incorporating a multifocus axis component into the design thereof to reduce the sensitivity of astigmatic patients to toric axis misalignment.
2. Discussion of the Related Art
Myopia or nearsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point before they reach the retina. Myopia generally occurs because the eyeball or globe is too long or the shape or contour of the cornea is too steep. A minus powered spherical lens may be utilized to correct myopia. Hyperopia or farsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point after they reach or behind the retina. Hyperopia generally occurs because the eyeball or globe is too short or the shape or contour of the cornea is too flat. A plus powered spherical lens may be utilized to correct hyperopia. Astigmatism is an optical or refractive defect in which an individual's vision is blurred due to the inability of the eye to focus a point object into a focused image on the retina. Unlike myopia and/or hyperopia, astigmatism is unrelated to globe size or corneal steepness, but rather it is caused by an abnormal, non-rotationally symmetric or non-spherical curvature of the cornea. A perfect cornea is spherical whereas in an individual with astigmatism, the cornea is not spherical. In other words, the cornea is actually more curved or steeper in one direction than another, thereby causing an image to be stretched out rather than focused to a point. A cylindrical lens rather than a spherical lens may be utilized to resolve astigmatism.
A toric lens is an optical element having two different powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical, for correcting myopia or hyperopia and one power, cylinder, for correcting astigmatism built into a single lens. These powers are created with curvatures at different angles which are preferably maintained relative to the eye. Toric lenses may be utilized in eyeglasses, intraocular lenses and contact lenses. The toric lenses used in eyeglasses and intraocular lenses are held fixed relative to the eye thereby always providing optimal vision correction. However, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction. Accordingly, currently utilized toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around.
When a toric contact lens is first placed in the eye, it must automatically position or auto-position itself and it then maintains that position over time. However, once the toric contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking as well as eyelid and tear film movement. Maintenance of the on-eye orientation of a toric contact lens is generally accomplished by altering the mechanical characteristics of the toric contact lens. For example, prism stabilization, including decentering of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge are all methods that have been utilized.
Each of more traditional stabilization techniques have advantages and disadvantages associated therewith. The main disadvantage of these types of designs is that they rely on the interaction of the eyelids and the contact lens' thickness differential to orient the contact lens to the correct location on the wearer's eye. The problem is particularly acute with so-called plus powered toric contact lenses.
An additional disadvantage associated with currently utilized toric contact lenses is the relatively large number of cylindrical axis placements required to fit astigmatic patients. In other words, in order to accommodate astigmatic patients utilizing toric contact lenses, a large number of stock keeping units (SKUs) are required.
Accordingly, it would be advantageous to design a toric contact lens with reduced reliance on rotational stabilization requirements as well as adding additional cylindrical powers to provide enhanced visual acuity.